HalfOfYear plugin

I miss having a "HalfOfYear" plugin, akin to "QuarterOfYear", that would let us work with halves rather than quarters.
A half goes from Jan 1 to June 30 (1), then from July 1st to December 31st.

Implementing this should be straightforward by replicating most of the logic for the QuarterOfYear plugin.

Would there be broader interest in this feature?

Start by locating the existing QuarterOfYear plugin and reading its implementation and tests. Add a corresponding HalfOfYear plugin that represents January 1–June 30 as 1 and July 1–December 31 as 2, then verify the plugin's date operations and formatting behavior with tests matching the quarter plugin.

Written by the indexing model from the issue text.